Receives up to maxlen bytes from udpsocket using
recvfrom(2) after O_NONBLOCK is set for the underlying file descriptor. If
maxlen is omitted, its default value is 65536. flags is
zero or more of the MSG_ options. The first element of the
results, mesg, is the data received. The second element,
sender_inet_addr, is an array to represent the sender address.

When recvfrom(2) returns 0, Socket#recvfrom_nonblock returns an empty
string as data. It means an empty packet.